Carbon steel tubes are widely used not only for piping but also for heat exchanger tubes in a heat exchanger and the like. Since carbon steel tubes used for such applications corrode because of the exposure to aqueous solution, they are generally processed by corrosion inhibiting, namely, anticorrosion treatment. The anticorrosion treatment is carried out in various ways. In case of a cooling water system, a method of adding corrosion inhibitors, namely, anticorrosives into the water system is generally used. As the anticorrosives to be added into the water system, phosphoric acid and/or phosphate (hereinafter, referred to as “phosphate”) base anticorrosives such as orthophosphoric acid, poly phosphoric acid, and phosphonic acid and zinc salt are widely employed. The addition of the anticorrosives forms a protective film on the surface of a metallic member such as a carbon steel tube, thereby inhibiting corrosion.
At the start of flowing water into a carbon steel tube which is newly installed and is not coated with anticorrosion coating or at the restart of flowing water into a carbon steel tube of which anticorrosion film is broken due to annual shut down, the carbon steel tube is treated to have a strong initial protective film formed thereon by adding anticorrosives in concentrated amounts into a water system in order to prevent corrosion just after starting or restarting water flow and to maintain a stable corrosion inhibition effect after that.
Conventionally, the treatment for forming an initial protective film is conducted by adding phosphate base anticorrosives or zinc salt in concentrated amounts into a-water system (Takahashi et al.: Water Re-use Technology, Vol. 14, No. 3, page 5 (1988), JP 2003-105573A). A film formed by the treatment for forming an initial protective film by using phosphate and zinc salt has a double layer structure composed of a precipitated layer made of P, Zn, Ca, O as the outer layer and a layer made mainly of iron oxide as the inner layer. Because of this double layer structure, the layer exhibits high anticorrosion effect (Kuniyuki Takahashi; corrosion inhibition '95 collection of lectures, A-305 (1995)).
Since there is the Environmental Standard which is 20 μg/L of zinc salt in a general sea water area, however, discharge of water containing zinc salt in concentrated amounts must be restricted. Accordingly, a corrosion inhibition method without using zinc salt is desired.
As a treatment for forming an initial protective film without using zinc salt, a method using anticorrosives of phosphate base such as sodium hexametaphosphate, with the total phosphate concentration being 100 mg-PO4/L has been put to practical use. However, such a treatment has a problem that the phosphate flows into a river, a lake, and/or ocean, thus causing eutrophication of water quality. Therefore, phosphate use is also regulated. It is desired to treat with a phosphate concentration as low as possible.
As anticorrosives for forming an initial protective film containing neither phosphate base anticorrosives nor zinc salt base anticorrosives, a water treating agent composed of water soluble aluminate and a specified ethylenic unsaturated carboxylic acid based copolymer containing hydroxyl group has been known (JP 2000-5742A). Though this water treating agent enables formation of an initial protective film without using phosphate base anticorrosives and zinc salt base anticorrosives, aluminate component contained in the agent and silicate component contained in the water system cooperate together to produce gel substrates during a process increasing the concentration of the water system after the treatment for forming an initial protective film according to water condition, operation condition, or the like and the gel substrates adhere the surface of the metallic member. The gel substances sometimes induce corrosion. This means that the anticorrosion treatment using the water treating agent is not necessarily stable.